CIS-PREFERENTIAL REPLICATION OF THE TURNIP YELLOW MOSAIC-VIRUS RNA GENOME

The largest open reading frame of the turnip yellow mosaic virus RNA g enome encodes a 206-kDa protein that is cleaved to yield N-terminal 15 0-kDa (p150) and C-terminal 70-kDa (p70) proteins. Using a genomic cDN A done capable of generating infectious transcripts in vitro, we have introduced substitution, frameshift, and in-frame deletion mutations i nto the regions encoding both proteins. None of the mutant RNAs was ab le to replicate independently in turnip protoplasts, indicating that p 150 and p70 are both essential. The replication in protoplasts of most of these defective RNAs was poorly supported in trans by a coinoculat ed helper genome with a deletion in the coat protein gene; replication could also be supported in trans by certain defective RNAs, but this complementation was likewise inefficient in most cases. The replicatio n in trans was more efficient for defective RNAs encoding wild-type p1 50 and defective p70 than for those encoding defective p150 and wild-t ype p70. One defective RNA with a large deletion in the p70 coding reg ion was able to replicate efficiently, both when inoculated with the h elper genome and when inoculated with a second complementing defective RNA that supplied a wild-type p70. Thus, the cis preference of replic ation can be overcome in some cases. A model in which p150 and p70 for m a complex with the 3' end of the RNA is proposed to explain the cis- preferential replication of turnip yellow mosaic virus RNA.